Update
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The Captain passed through the metal detector and got green light. Security guards said he walked too fast and asked him to go through again. He did and got green again. Guards still maintained he had walked too fast and asked him to pass through for the thrid time.
He refused.
The AC had started taxiing when it was called back by the tower and told to return to the gate. It did. Once there, pilots requested and got new clearance, which they recieved. Taxied out again and were called back again.
Pax off loaded.
The company's union rep states that the Captain concerned does not recognise the description of the event as given by the security staff, and that the whole thing is a storm in a teacup.
The company refuses to comment beyond saying that they want to get to the bottom of this and that the Captain is grounded until they do.